Emily Mortimer - Naughty, but ever so nice

Emily Mortimer is telling me about Martin Scorsese. "The conversation goes on for hours, because he can't stop talking. You never feel you're boring the great auteur, or 'Oh God, I shouldn't have said that!' It's just so amazing, why can't all geniuses be like this?," she laughs.

Mortimer has just shot Scorsese's new film, the feverishly awaited Shutter Island, alongside Leonardo DiCaprio, Michelle Williams, Mark Ruffalo and Ben Kingsley. A 1950s crime story based on a Dennis Lehane novel, it's about the hunt for a murderess who has escaped from a hospital for the insane on an island in Massachusetts. The plot of the film is a secret, but Mortimer is effusive about Scorsese.

"His motivation comes from his love of film. That's the thing he cannot stop talking about: other people's movies. Everything he does, every shot, is a homage to someone else, such as Hitchcock. He's getting such a kick out of it."

People always claim Mortimer reminds them of the naughtiest girl in the school. She's tall and skinny, with long dark hair and wicked, dark eyes. Unapologetically sweary, she revels in the English art of self-deprecation. But when I meet her, she is noticeably more grown-up. And she's been working with an impressive list of auteurs, from Scorsese to Woody Allen and David Mamet.

Once Mortimer was seen as a posh English rose. The daughter of the author, playwright and barrister John Mortimer and his second wife, Penny, she had a privileged childhood. They spent summers in the south of France at the house of the late director Tony Richardson, with his daughters (by his first wife, Vanessa Redgrave) Joely and Natasha. After St Paul's Girls School in London, she studied English and Russian at Oxford. She never went to drama school. A producer saw her in a college play, and started sending her for castings.

Her first professional role was the 1995 Catherine Cookson mini-series The Glass Virgin. More corset parts followed in Bright Young Things, Ken Branagh's Love's Labour's Lost and Elizabeth. She wrote a Bridget Jones-style newspaper column in her mid-twenties and played the "perfect girl" Hugh Grant couldn't fall in love with in Notting Hill. But mostly she was known as a famous daddy's girl.

But then she started doing quirky independent films – including David Mackenzie's Young Adam (where she was spanked and covered in custard by Ewan McGregor) and Nicole's Holofcener's Lovely and Amazing (where she shed her clothes for a full-frontal nude scene).

Allen snapped her up for the gruesome posh girl who marries a social-climbing rat in Match Point. She was terrific and sent up every affectation you might have attributed to her. "Bloody hell," you thought. "She really can act." Then came her wonderfully touching role in Lars and the Real Girl – where she played the pregnant sister-in-law who supports Ryan Gosling when he falls in love with a blow-up doll.

At that point, the auteurs started calling, including Mamet, who cast her as a troubled lawyer in his latest martial-arts action-drama, Redbelt. She was expecting Mamet to be terrifying: "But I really loved him. The film is set in this very macho world but he's a fruity old luvvie, really. He's incredibly encouraging and kind."

In fact, the only director she found puzzling was Allen. Shooting Match Point, he'd happily gossip about great Indian restaurants in London, but work was taboo. "He doesn't have any chat about the job in hand. You bring your interpretation of the character and he just does a couple of takes and everyone goes home for tea. It's kind of brilliant but really hard. Maybe that confusion helps, because you have to go on your instincts." She jokes that she'd always hoped to play a neurotic Allen heroine "and then I'm cast as a silly posh bitch".

Not any more. The next film you'll see her in is the brilliant Transsiberian, a thriller set in a Siberian wasteland. It's directed by Brad Anderson (The Machinist) and horror fans are blogging about it. Mortimer and Woody Harrelson play the all-American couple who find out what they are capable of. Mortimer gets to bludgeon a man to death, hang off a moving train, and there's a torture scene. "You think: 'Cor, Brad, he's got some weird stuff going on in his head,'" she deadpans.

"What I love about Jessie is that she's a good person, and at the same time she has a need to put her hand in the fire. I relate to that. I know I seem like butter wouldn't melt, but I have the capability to be awful and have all these dark thoughts."

Ironically, Mortimer's success comes at the very moment she was dreading. "I was good at being quite flighty and dippy. But I didn't feel so convincing as a the mature, sensible woman. I can see it with my little boy, Sam, who is five. Just as he feels the excitement of mastering a new stage of his life, at the same time he becomes needy."

Mortimer is based in New York with her husband, the American actor Alessandro Nivola, whom she met filming Love's Labour's Lost. She misses her parents, especially now her father is wheelchair-bound and almost blind. "He's been the most wonderful dad. Being with him now he's 85 is still more fun than being with most people."

But Nivola has given her stability. He coaxed her out of her English embarrassment at ambition. "He said, 'What are you doing, you seem to be embarrassed by your job.'" She realised she had been pretending acting was a way of justifying her expensive education. Now she takes it more seriously.

She's been making efforts to assimilate in America, including a stint on Broadway in the Jez Butterworth play, Parlour Song (which transfers to London next year). And she's been volunteering as a prison visitor. "The striking thing is, they're just like you and me. They are normal girls. That's one of the most important things my dad has given me, this open-mindedness about the difference between right and wrong; how hard it is to really tell."

The first child of her father's second marriage, born when he was 48, she was an only child until the birth of her younger sister 12 years later. "It is a really good thing for a girl to be loved by her dad," she has said. "It makes you confident with men. And that is an amazing gift."

Her own screenplay – an adaptation of Lorna Sage's childhood memoir Bad Blood – goes into production next year, directed by Matthew Warchus. She tells me excitedly that Julie Walters may be playing the grandmother.

Mortimer never met Sage, but is humbled by the writer's life, "which makes me feel incredibly Sloaney in comparison". Though she adds that one of the advantages of getting older is you realise you, too, have faced your own share of battles.

Four years ago, just before the publication of Graham Lord's biography of John Mortimer, she discovered she had a half-brother from her father's affair with the actress Wendy Craig. They had a bruising time from the tabloids, but she says now: "We all adore him and he's made us closer as a family."

She will be back filming here in January – in Harry Brown, opposite Michael Caine as a vigilante pensioner. "When I come back to London it takes me a couple of days to get my mean factor going. Then I'm as good as the rest of them."

'Redbelt' is out now; 'Transsiberian' opens this autumn